Significance Well-being assumes the ability to carry out basic biological functions in a normal fashion and in the absence of persistent symptoms of stress or disease. Understanding those factors has broad implications for biomedical research as well as implications for care and maintenance of laboratory animals. Objectives To evaluate the social living environment that impact well-being of New World monkeys by examining activity and reactivity of stress response systems, reproductive processes and health status. Our objectives include understanding basic physiological and behavioral processes as well as identification of the optimal living arrangements for the promotion of well-being in these animals. Results Squirrel monkeys exhibit prolonged periods of reduced cortisol levels following group formation. This is analogous to patients suffering from post-traumatic stress disorder and inimical to well-being. Several months of cohabitation are required for groups to stabilize and pituitary-adrenal activity to normalize. Individual housing in this species does not produce symptoms of chronic stress. Titi monkeys, in contrast, adapt readily to new social configurations. Formation of new pairs results in an elevation in cortisol, but heightened adrenocortical activity persists for less than 48 hours. Individual housing for titi monkeys results in persistent elevations in cortisol characteristic of bereavement or depression. Studies of reproduction show that titi monkey females exhibit lactation induced suppression of ovulation and they usually conceive on their only ovulatory cycle of the year. Interbirth intervals are remarkably consistent in titi monkeys but not constrained to an annual cycle. Interbirth intervals in squirrel monkeys are more variable and constrained by a rigid annual cycle. Future Directions Examining changes in stress responsiveness at various stages in the reproductive process. We are particularly interested in neuroendocrine changes surrounding parturition that may suppress stress responsiveness to many environmental stimuli but enhance responsiveness to infant related stimuli. KEY WORDS pituitary adrenal activity, social influences, reproduction, pituitary-gonadal activity, aggression FUNDING NIH Grant RR00169